SpaceX Super Heavy SSTO and Starship Depot

 

Booster 7 in testing

It may surprise some people but it's kind of known within the online space community that the SpaceX Super Heavy Booster, at least on paper, has the numbers to be a Single Stage To Orbit (SSTO) vehicle. I am pretty amazed by this and wanted to understand it a bit better. So here are the DeltaV calculations (I have used where I can reported figures for Raptor 2 and Starship/Super Heavy). Raptor 2, 327/363 ISP, 230t thrust, 1.6t weight, Super Heavy 3650t, 3400t fuel.

If you add the gravity boost for launching east (460ms) you can see the booster alone can make Low Earth Orbit (LEO). Of course it would need modifications like a nosecone to actually achieve this which will add some weight. It does not have the reserves to land back on Earth from orbital velocity. Which is good because it also does not have the necessary shielding or flight profile as it was never intended to land from orbit. This means it's an expended launcher. 

We can actually make these numbers better for Super Heavy SSTO. 33 engines is necessary to launch the whole Super Heavy and Starship stack. The Thrust to Weight Ratio (TWR) is too high for Super Heavy alone. If Raptor 2 has a reported thrust of 230 tons then we only need about 21 engines. We could remove 12 engines, reducing weight by about 20 tons and increasing payload to 20 tons.

Aeneas has modelled another variation that can achieve 100t to orbit expended. He replaces some Sea Level Raptors with Vacuum Raptors. I think, with some modification and a bit of weight penalty we could turn his version into a large Starship variant and bring it back for re-use. A reusable SSTO launcher that can put more than 40t in orbit would be fantastic, as Elon Musk says, the best part is no part and if optimising for LEO delivery this is sort of the perfect vehicle.

  

So does this mean that SpaceX should abandon Starship and just use Super Heavy to launch loads into space? While it could possibly be a short term option, the cost of building and launching 20 - 100 tons via an expendable Super Heavy would put it into the same sort of cost to orbit bracket as Falcon Heavy. Estimates put the cost of building a Super Heavy booster at as much as $100 million currently. An expended Falcon Heavy costs the customer $150 million (much less for SpaceX) and can put 68 tons in orbit. That doesn't really justify the 10 billion spent so far on the Starship program to make cost to orbit cheaper. Without reusability it would slow the cadence of launching considerably and keep costs high. However with reusability it could very much be worthwhile for launching LEO payloads.

Falcon Heavy launches on it's demo mission

If it were expendable it would not justify the project costs to make it an average value launch vehicle what could we use it for then?

Super Heavy itself has almost 10kms DeltaV if refueled in orbit. That's an incredible amount of boost for a high energy mission like going to Mars or Jupiter in a short time frame. @BellikOzan on X has imagined some scenarios around this. It's an incredible opportunity for science and manned missions to have that sort of energy available in orbit.

One other possibility is around the Depot ship for refueling in LEO. Currently SpaceX has plans for a stretched Starship in this role. But even doubling the size of Starships tanks falls far short of a Super heavy in orbit, 2400t vs 3400t, even assuming you use up the payload capacity as well, it is probably still far short.

There is one wilder possibility, as an optimised Super Heavy can put approx 100t in orbit and the Orion plus ICPS weighs 65t then NASA could save a couple of billion and launch on a bit more optimised Super Heavy SSTO. It's never happening, but a fun thought experiment.

So in short a Super Heavy SSTO could be something we might desire to have in the future. Expendable it would only be used infrequently, if it was made reusable it could be a bright star.